The present invention relates generally to an apparatus and method for the controlled application of heat to develop a predetermined temperature for fusing items made of thermoplastic material.
It is known to fuse thermoplastic pipes together by means of a cylindrical thermoplastic coupling containing a heating coil. The pipes are inserted into the coupling, defining an interface between the pipes and the coupling. The coil is energized, providing heat which raises the temperature of the pipes and the coupling at the interface to a temperature that is high enough to fuse the coupling and the pipes together.
The heat must be applied for a sufficient time to raise the temperature throughout the interface to the fusion temperature so as to fuse the pipes and the coupling into a mechanically strong, leakproof connection. If not enough heat is applied, portions of the interface will never reach the fusion temperature and the fusion will be incomplete, resulting in a leaky connection. Conversely, if too much heat is applied, the heat will diffuse all the way through the pipes and melt them. In addition, if the heat is applied at too great a rate, hot spots will occur in the thermoplastic material adjacent the heating coil, resulting in localized melting of the thermoplastic, short circuits between adjacent turns of the coil, and possibly fire. Consequently, it is necessary to precisely control the application of heat so that just enough heat is applied at a sufficiently rapid rate to raise the temperature of the pipes and the coupling throughout the interface to the fusion temperature at substantially the same time without melting the pipes or causing hot spots.
The amount of heat which must be applied is a function of the ambient temperature. Non-uniform heat diffusion is a particular problem when the ambient temperature is low, as in winter. In addition, the required amount of heat is also a function of the diameter of the pipes, more heat being required if the pipes are large than if they are small.
Accordingly, there is a need for a way to control the rate of application of heat so as to attain fusion temperature at substantially the same time throughout an interface between thermoplastic materials without damaging the materials.